The use of quartz crystal or tuning fork quartz oscillators in gas friction manometers is known. Two measuring effects can be utilized for such applications because, as a function of the ambient pressure, such quartz oscillators can both have a frequency dependency and be subject to damping. For a good measurement accuracy in low pressure ranges (pressure&lt;1 mbar) the damping of the oscillating quartz is preferably determined by measuring the resistance at resonant frequency, because the pressure-dependent change to the resonant frequency at low pressures is too small in order to permit reliable evaluation by measurement.
EP-A-No. 0 233 054 discloses a gas pressure sensor, which exposes a tuning fork quartz to the measuring environment. The aforementioned principle of the resistance measurement of a tuning fork quartz at resonant frequency is used. By means of a phase-coupled, electric oscillation system (PLL circuit) and an evaluation circuit, the change of resistance of the tuning fork is measured and appropriately indicated.
Although the corresponding measuring principle has been known for a considerable time, the practical use of such tuning fork quartz sensors has hitherto been restricted or made impossible by numerous different problems. A major problem in connection with this measuring method is that the measured damping value is not only dependent on the pressure, but also on the temperature. This is negligible at high pressures (&gt;1 mbar). However, the desired measurement can be greatly impaired by the temperature dependence at lower pressures. In order to reduce the interference effects, a number of possibilities has been proposed. Thus, e.g. the apparatus according to EP-A-No. 0 233 054 uses a special, tailor-made oscillating quartz in order to keep the temperature influences as low as possible. It has also been proposed in connection with tuning fork quartzes to use a negative temperature coefficient, i.e. a NTC-resistor, which when connected in series brings about a temperature compensation (cf. EP-A-No. 0 180 297). Another possibility consists of thermostatically controlling the quartz (cf. Hirata M. et al, J. Vac.Sci.Technol., A5, 2393, 1987).
Another important problem results from the damping of the tuning fork quartz caused by the mounting support of the quartz. A poorly defined mounting support leads to imprecise or unreproducible measured values and only very limited power is converted in the said quartz. This makes the measurement very susceptible to error. The error may, particularly, result from the conditions in the measuring volume. Considerable significance is also attached to direct gas impacts acting on the quartz or contaminant or dirt particles, which make it necessary to appropriately position the quartz.
Use of a special quartz geometry, as proposed in EP-A-No. 0 233 054 can lead to an improvement in the measurement characteristics of the quartz, but does not satisfy the practical requirements. The use of such special quartzes also leads to higher manufacturing costs. It is desirable that standard tuning fork quartzes, such as are used as time-maintaining elements in watches and clocks, should be usable also in manometers.
Compensation of the temperature influences by means of a NTC-resistor connected in series with the quartz oscillator, as proposed in EP A-No. 0 180 297, only leads to inadequate results, because there are only a few degrees of freedom for the compensation and in particular higher order components remain uncompensated. Further, thermostatic control leads to very high, unacceptable costs. Thus, the proposals made up to now only lead to a very limited error or fault compensation, whilst at the same time being complicated and costly.
Account has not been taken in the conventional damping measurement circuits, e.g. that of U.S. Pat. No. 4,507,970 of the fact that the output valve, or quantity is substantially linearly dependent on the damping. In the case of vacuum measurement generally a constant relative precision is desired over the pressure range to be measured. In the case of conventional circuits when measuring low pressures very small damping and therefore output quantity changes have to be resolved. This leads to an additional high interference susceptibility at low pressures.